A trap set by the Fish and Wildlife Service catches all fish that pass through this stream. Biologists return the Arctic grayling to the waterway, while the more numerous and nonnative cutthroat trout are killed for study and later donated to a local food bank.

Trapped cutthroat trout are gutted at the Fish and Wildlife field station in the Centennial Valley. Their stomach contents are studied to determine their diet and whether they are outcompeting the native Arctic grayling.

Conservationists inadvertently depleted Arctic grayling populations in Montana's Centennial Valley when they dammed streams there to save habitat for the trumpeter swan.

James M. McCannScience Source

To keep America's wilderness anything like it used to be when the country was truly wild takes the help of biologists. They have to balance the needs of wildlife with those of cattle-ranching and tourism, and even weigh the value of one species against another. Ultimately, they have to pick and choose who makes it onto the ark. And, as scientists in Montana's Centennial Valley have discovered, all that choosing can be tricky.

Take the case of the valley's trumpeter swans. These are the largest waterfowl in North America. They have a 7-foot wingspan. They're ivory-white, curvaceous, and elegant — and 80 years ago they were almost extinct. Simply put, there were too many people using the same land the swans needed. And there were too many hunters.

The flock of trumpeters I found lounging on a lake in the Centennial Valley belongs to a small population that has struggled back from the brink. In the Centennial, biologists built ponds for the birds, and fed them — and the swans' numbers recovered.

Unfortunately, what was good for the swans was not so good for the Arctic grayling, another rare species in the valley.

What happened was this: To make the swan-friendly valley even more so, the biologists created ponds by damming streams. When they did that, the grayling lost streambeds they'd been using for egg-laying and reproduction.

The lost streams were much like Red Rock Creek, a sparkling, braided rivulet that eventually becomes the Missouri River. "This is where the majority of the Arctic grayling in the Centennial Valley spawn," explains Nathan Korb, who has brought me through tall grass along a path to the stream's edge.

Korb is a compact, athletic guy with shoulder-length blond hair and a quick smile. He lives and works in the valley most of the year and works for The Nature Conservancy. Scientists, he acknowledges, sometimes make mistakes.

"They did everything that they knew was best for those birds," Korb says, "but now we have a much broader perspective, so we are thinking about all species. There are lots of examples where we try something that sounds like a good idea, [and it] turns out not to be that good of an idea. Then [we] remedy it and — hopefully — never try it again."

Now scientists here are trying to fix things for the grayling in Montana.

Korb takes me to a weir that straddles the narrow stream. It's a trap to catch trout, especially the cutthroat trout, an aggressive intruder. The cutthroat appears to be pushing the grayling out.

The weir is basically a pole-and-screen fence that straddles the stream so fish can't move upstream or downstream unless they go through the one opening, and get trapped. Then biologists collect them — unless a bear gets there first.

The cutthroat isn't native here; the grayling is. The grayling is distinctive, with its large and colorful dorsal fin. While the cutthroat is widespread and aggressive, the rarer grayling is known as the "lady of the stream." In fact, this is the only population of the lake-dwelling Arctic grayling left in the Lower 48 states. The federal government may soon add the fish to its list of endangered species.

We drive up a muddy road to see the next step in the experiment to save the grayling, at the headquarters of the Red Rock Lakes National Wildlife Refuge.

Refuge manager Bill West is gutting and cleaning a bucketload of cutthroat trout that have come from the stream trap. "I want to pass on definitively to the next manager whether the cutthroat are part of the problem or are just benign," he says.

Figuring that out isn't easy. When it comes to managing wildlife, each situation is unique: This fish in this valley in these waters is unlike any other. One way to understand the fish better is to open it up. U.S. Fish and Wildlife biologist Kyle Cutting spends hours doing exactly that. "Just make one slit in the belly," he demonstrates on a metal table pooled with blood and fish guts. "You hit the spine and break it through, and we just remove the insides."

Cutting is trying to find out what the cutthroats are eating. Is it the same stuff the grayling eat? Are they pushing the grayling out? For sure, the cutthroats live up to their name. "In the spring and late fall, their stomachs were just packed full, and oftentimes these cutthroat had fish hanging out of their gills on both sides — you know, 7-inch-long fish."

The cutthroat trout the biologists kill for examination aren't wasted — they end up in local food banks. And the information they provide will help the wildlife managers decide the best way to help the grayling without tipping the ecological balance against yet another native species.

Biologists in Montana's Centennial Valley acknowledge they are micromanaging, and that their best efforts derive from an imperfect science. If they forget that, for even a minute, they have the Trumpeter Swan Society to remind them. The society has already warned that the new focus on trout should not jeopardize the swan's tenuous comeback.

The society's Ruth Shea says the trumpeter population in the valley is not out of danger yet and shouldn't be neglected. "Centennial Valley is the single most important breeding area for these swans in the U.S.," she says. "It's huge." Wildlife managers will have to plan carefully to keep both swan and grayling in the ark.

Montana is home to a big chunk of American wilderness and to animals that need it to survive. To help preserve the wilderness, biologists have to balance the needs of wildlife with cattle ranching and tourism, and even one species against another. As NPR's Christopher Joyce reports in the second part of our series on Centennial Valley, managing wildlife is a tricky science, just like picking and choosing what makes it onto the ark.

CHRISTOPHER JOYCE, BYLINE: Trumpeter swans have a seven foot wingspan. They're ivory-white, curvaceous, elegant and 70 years ago they were almost extinct. Too many people, and especially, too many hunters. I found a flock of these swans lounging on a lake in the Centennial Valley. Biologists built ponds for them here, even fed them and the swans' numbers recovered.

Unfortunately, what was good for the swans was not so good for the Arctic grayling trout, another rare species in the valley. What happened was biologists wanted to keep swans in the valley. It was safe from hunters. So they made ponds by damming streams. When they did that, the grayling trout apparently lost streambeds they lay eggs and reproduce. Steams like this one.

NATHAN KORB: So is Red Rock Creek. This is where the majority of the Arctic grayling in the Centennial Valley spawn.

JOYCE: Ecologist Nathan Korb is with The Nature Conservancy. Scientists, he says, sometimes make mistakes.

KORB: They did everything that they knew was best for those birds, but now we have a much broader perspective, so we are thinking about all species. There are lots of examples where we try something that sounds like a good idea, turns out not to be that good of an idea, and then remedy it and hopefully never try it again.

JOYCE: And now, they're trying to fix things for the grayling in Montana. Korb is a compact, athletic guy with shoulder-length blond hair and a quick smile. He pushes through tall grass to show me a weir that straddles the narrow stream. It's a trap to catch trout, especially an aggressive intruder, cutthroat trout. The cutthroat appears to be pushing the grayling out.

Well, what an elaborate-looking contraption that is.

KORB: Yeah, basically a pole fence that goes across the whole length so fish can't move up or downstream unless they go through the one opening.

JOYCE: The cutthroat is not native here. The grayling is. The cutthroat is widespread and aggressive, the grayling is not. In fact, this is the only population of the lake-dwelling Arctic grayling left in the Lower 48. It's distinctive with a large and colorful dorsal fin. The federal government may add it to the endangered species list.

We drive up a muddy road to see the next step in the grayling experiment at the headquarters of the Red Rock Lakes National Wildlife Refuge. Refuge manager Bill West is gutting and cleaning a bucket-load of cutthroat trout that have come from the stream trap.

BILL WEST: Some of the things that the refuge might have done to try to enhance swans in the past, build ponds, blocked these fish from their native spawning grounds.

JOYCE: Now, they want the streams under those ponds back and the plan to revive the trout population.

WEST: I want to pass on, definitively, to the next manager whether the cutthroat are part of the problem or they are just benign.

JOYCE: So biologists have to figure out how to figure that out. That's not easy. When it comes to managing wildlife, each situation is unique. This fish, in this valley, in these waters is unlike any other. One way to get to know it is to cut it open.

KYLE CUTTING: Just make one slit in the belly. You hit the spine and break it through, and we just remove the insides.

JOYCE: Biologist Kyle Cutting works for the U.S. Fish and Wildlife Service here. He's cutting open cutthroats to find out what they're eating. Is it the same stuff the grayling eat? Are they pushing the grayling out?

CUTTING: In the spring and late fall, their stomach were just packed full, and oftentimes these cutthroat had fish hanging out of their gills on both sides, you know, seven-inch-long fish.

JOYCE: The cutthroat trout here end up in local food banks. But the information they provide will help biologists decide the best way to help the grayling and avoid tipping the ecological balance against yet another native species. Biologists in Montana's Centennial Valley acknowledge that they have to micromanage wildlife this way and that it's an imperfect science. And in case they forget, they have the Trumpeter Swan Society to remind them. The society has already warned them that the new focus on trout should not jeopardize the swan's own tenuous comeback.

Christopher Joyce, NPR News.

GREENE: And you can see the wildlife in Montana's Centennial Valley at NPR.org. The images in our photo gallery were taken by NPR's John Poole. Transcript provided by NPR, Copyright NPR.